A group communication system (GCS) is a communications system with a scheduling capability. In the group communication system, data transmission may be performed in a unicast transmission manner or a multicast transmission manner. When data transmission is performed in a multicast transmission manner in group communication in Long Term Evolution (LTE), data transmission is usually performed by using a multimedia broadcast/multicast service (MBMS).
A GCS in LTE usually includes an evolved NodeB (eNB), a group communication service application server (GCS AS), and a broadcast/multicast service center (BM-SC). An eNB at a transmit end sends received data to the GCS AS. The GCS AS determines to send the received data to an eNB at a receive end in a unicast transmission manner or a multicast transmission manner. There is usually one eNB at the receive end in the unicast transmission manner, and there are usually multiple eNBs at the receive end in the multicast transmission manner. When the GCS AS determines to send the received data to the eNB at the receive end in the multicast transmission manner, the GCS AS sends the data to the BM-SC, and the BM-SC sends the data to multiple eNBs at the receive end in the multicast transmission manner.
When data transmission is performed in the multicast transmission manner, to ensure that data can be simultaneously transmitted to multiple eNBs at the receive end, when sending the data, the BM-SC needs to determine a relatively long synchronization cycle according to factors such as lengths of different paths between the BM-SC and the multiple eNBs at the receive end, so as to send the data. Consequently, a data transmission delay is relatively large. The synchronization cycle is a minimum time interval by using which the multiple eNBs at the receive end can receive the data sent by the BM-SC.